1. Field of the Invention
Example embodiments generally relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus incorporating the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such fixing device is requested to shorten a first print time taken to output the recording medium bearing the fixed toner image onto the outside of the image forming apparatus after the image forming apparatus receives a print job. Additionally, the fixing device is requested to generate a sufficient amount of heat even when a plurality of recording media is conveyed through the fixing device continuously at increased speed for high speed printing.
To address these requests, the fixing device may employ a thin endless fixing belt having a decreased thermal capacity that decreases an amount of heat required to heat the fixing belt to a given fixing temperature at which the toner image is fixed on the recording medium. FIG. 1 illustrates such fixing device 20R1 that incorporates a thin endless fixing belt 100. For example, as shown in FIG. 1, a pressing roller 400 is pressed against a substantially tubular, metal thermal conductor 200 disposed inside a loop formed by the fixing belt 100 to form a fixing nip N between the pressing roller 400 and the fixing belt 100. A heater 300 disposed inside the metal thermal conductor 200 heats the fixing belt 100 via the metal thermal conductor 200. As the pressing roller 400 and the fixing belt 100 rotate and convey a recording medium P bearing a toner image T through the fixing nip N in a recording medium conveyance direction A1, the fixing belt 100 and the pressing roller 400 apply heat and pressure to the recording medium P, thus fixing the toner image T on the recording medium P. Since the heater 300 heats the fixing belt 100 via the metal thermal conductor 200 that faces the entire inner circumferential surface of the fixing belt 100, the fixing belt 100 is heated to a given fixing temperature quickly, thus meeting the above-described requests of shortening the first print time and generating heat sufficiently.
However, in order to shorten the first print time further and save more energy, the fixing device is requested to heat the fixing belt 100 more efficiently. To address this request, a configuration to heat the fixing belt 100 directly, not via the metal thermal conductor 200, is proposed as shown in FIG. 2.
FIG. 2 illustrates a fixing device 20R2 in which the heater 300 heats the fixing belt 100 directly. Instead of the metal thermal conductor 200 depicted in FIG. 1, a nip formation plate 500 is disposed inside the loop formed by the fixing belt 100 and presses against the pressing roller 400 via the fixing belt 100 to form the fixing nip N between the fixing belt 100 and the pressing roller 400. Since the nip formation plate 500 does not encircle the heater 300 unlike the metal thermal conductor 200 depicted in FIG. 1, the heater 300 heats the fixing belt 100 directly, thus improving heating efficiency for heating the fixing belt 100 and thereby shortening the first print time further and saving more energy.
However, since the fixing belt 100 is heated by the heater 300 directly, the fixing belt 100 is subject to overheating that may result in deformation of the fixing belt 100 by thermal stress induced therein. For example, when the fixing belt 100 interrupts its rotation immediately after a print job is finished, residual heat remaining in the heater 300 is conducted to the fixing belt 100, thus heating a part of the fixing belt 100 disposed opposite the heater 300 directly. Accordingly, that part of the fixing belt 100 may overheat and deform. Consequently, the deformed fixing belt 100 may not apply heat and pressure to the recording medium P conveyed through the fixing nip N properly, resulting in faulty fixing.